The present invention relates to method and apparatus for providing a wavelength-locker arrangement which is wavelength independent and provides for the absolute wavelength stability of a laser diode.
Absolute wavelength accuracy of a laser diode is of paramount significance for the successful deployment of a practical dense wavelength division multiplexed (WDM) transmission system. However, due to the aging of a laser and such phenomenon, laser wavelength shifts with time. This places an undue restriction on the remaining components of a WDM transmission system. Some of these laser phenomenon issues are alleviated by the use of a wave-locker, which monitors the laser wavelength and actively changes the temperature of the device which mounts the laser in order to compensate for any wavelength drifts. However, state of the art wave-lockers are constructed using micro-optic filters or etalons which add a significant cost and size to a laser diode device. In addition, such wave-lockers have high insertion loss which leads to a reduced signal-to-noise ratio due to the use of a prior optical tap in the system. Therefore, it is of significant advantage to construct an all-fiber device which is capable of limiting the wavelength drift in a laser diode without adding significant cost.
The concept of stabilizing the wavelength of a laser diode is based on the ability to calibrate absolute wavelength drifts into measurable absolute power changes. One way,.this is accomplished is by tapping a small fraction of the laser""s optical output signal and sending the signal to a filter with a wavelength dependent response.
Referring now to FIG. 1, there is graphically shown an exemplary filter profile 10 in a transmission laser signal. A transmission value (dB) is shown along the vertical axis and wavelength in nanometers (nm) is shown along the horizontal axis. If a filter is designed such that the laser""s center wavelength (laser line) is aligned along the edge of the filter in the manner shown in FIG. 1, it is possible to translate the wavelength information into power information. For example, it is seen in FIG. 1 that as the wavelength of the laser increases, the power at the output of the filter will increase proportionally. However, such an arrangement by itself is not adequate to ensure absolute stability of the laser diode device because a change in the power of the laser diode can be misconstrued as a wavelength change. To overcome this problem, the response of the filter has to be monitored in reflection as well.
Referring now to FIG. 2, there is graphically shown an exemplary laser diode signal where a solid line 12 shows the laser transmission through a filter, and a dashed line 14 shows the laser transmission in reflection from the filter. As is shown, the reflection response 14 of the filter is complementary to the transmission response 12 of the laser diode so that an increasing wavelength will result with a decrease in the reflected power. Since a filter""s transfer function is known apriori, the ratio of the transmitted to reflected power can be used to calibrate any power drifts. Finally, xe2x80x9cyxe2x80x9d tracking of the response of the filter, and changing the temperature of the laser, will hold the wavelength of the laser diode at a constant value. Two classes of wave-lockers have been commercially developed using the principle of operation described above. A first class is a filter-based wave-locker, and a second class is an etalon-based wave-locker.
Referring now to FIG. 3, there is shown a block diagram of an exemplary prior art filter-based wave-locker arrangement 20 (shown within a dashed line rectangle) illustrating the first class of wave-locker. The wave-locker arrangement 20 comprises a laser source (LASER) 22, an optical power tap 24, a wave-locking device 26 (shown within a dashed line rectangle) and a control unit (CONTROL) 28. The optical output from the laser source 22 is received at an input of the power tap 24. The power tap 24 divides the received laser signal into two portions, where a first portion of the laser signal is provided as an output of the wave-locker arrangement 20, and a second portion of the laser signal is coupled to an input of the wave-locking device 26. The control unit 28 receives first and second outputs from the wave-locking device 26 and generates therefrom appropriate control signals to the laser source 22 for maintaining its wavelength at a substantially constant value.
The wave-locking device 26 comprises a wide-band power splitter (PWR.SPLIT.) 30, a wavelength discriminating filter (FILTER) 32, a first photodetector (PHOTO DETECT) 34, and a second photodetector (PHOTO DETECT) 36. An exemplary optical response of the filter 32 is shown in FIG. 4 where the vertical axis denotes transmission (dB) and the horizontal axis denotes wavelength (nm) in the manner of the graph of FIG. 1. The second portion of the laser signal outputted from the power tap 24 is received at a first terminal of the wide-band power splitter 30, a second terminal of the wide-band power splitter 30 is coupled to an input of the filter 32, a third terminal of the wide-band power splitter 30 is coupled to an input of the second photodetector 36, and a fourth terminal of the wide-band power splitter 30 is unused or pig-tailed. An output of the filter 32 is coupled to an input of the first photodetector 34, and outputs from the first and second photodetectors 34 and 36 arc provided as electrical feedback signals to a control unit 28 which generates control signals to actively control the wavelength of the laser source 22.
In operation, a fraction of light from a laser source 22 (typically 5%) is tapped off by the power tap 24 and sent to the wave-locking device 26. In the wave-locking device 26, the received fractional signal is split into two portions by the power splitter 30. A first portion of the split signal is sent to the wavelength discriminating filter 32, and a second portion of the split signal is effectively unused by being directed to the fourth terminal of the power splitter 30. At the filter 32, a part of the signal is reflected back through the power splitter 30 to the second photodetector 36, and the remaining part of the received signal is transmitted to the first photodetector 34. Each of the first and second photodetectors 34 and 36 generate electrical output signals corresponding to the received input signals. The two electrical signals from the first and second photodetectors 34 and 36 are provided as feedback signals to the control unit 28. The control unit 28 uses the two feedback signals to generate control signals to the laser source 22 to monitor and control its wavelength.
The filter-based wave-locker arrangement 20 has two significant disadvantages. First, since the filter 32 is wavelength selective, it can only be used at a specific International Telecommunication Union (ITU) recommended wavelength. More particularly, the ITU has recommended a wavelength range of 1550-1576 nanometers (nm), which is called a 1550 window, within a grid or scale that has a reference frequency of 193.1 terahertz and 50 GHz intervals. For example, in a typical 50 GHz operation, in the 1550 nm window alone there are over 100 useable wavelengths, and such a device would require fabrication of over 100 different filters leading to cost and inventory issues. Second, due to the need for splitting the incoming power to the wave-locking device 26 in the power splitter 30 so as to be able to access both the reflective and transmitted signal, the actual signal reaching the photodetectors 34 and 36 is very small. Specifically, there is a loss of 3 dB for the transmitted signal and a 6 dB loss for the reflected signal, and the signal-to-noise ratio suffers significantly leading to errors in the control unit 28.
Referring now to FIG. 5, there is shown a block diagram of an exemplary prior art etalon-based wave-locker arrangement 40 (shown within a dashed line rectangle), illustrating the second class of wave-locker. The wave-locker arrangement 40 comprises a basic optical circuit that is similar to the one shown in FIG. 3, but the filter 34 is replaced by an etalon 42. More particularly, the wave-locker arrangement 40 comprises a laser source (LASER) 22, an optical power tap 24, a wave-locking device 46 (shown within a dashed line rectangle), and a control unit 28. The wave-locking device 46 comprises a wide-band power splitter (PWR. SPLIT.) 30, an etalon 42, a first photodetector (PHOTO DETECT) 34, and a second photodetector (PHOTO DETECT) 36. The elements of the wave-locker arrangement 40 having the same designation number as those in the wave-locker arrangement 20 of FIG. 3 have corresponding functions as described hereinbefore. An optical response of the etalon 42 is shown in FIG. 6 where the vertical axis denotes transmission (dB) and the horizontal axis denotes wavelength (nm) in the manner of the graphs of FIGS. 1 and 4. By appropriate design of the etalon 42, it can be ensured that the wavelength response is periodic and consistent with an ITU grid. This obviates the need of a unique wave-locker at every wavelength, but still suffers from the need of having a wide-band 3 dB power splitter 30, high insertion loss, and consequently a low signal-to-noise ratio at the first and second photodetectors 34 and 36. Additionally, both of the wave-locker arrangements 20 and 40 require a filter 32 or an etalon 42, and a wide-band power splitter 30 making the device footprint large and expensive. Still further, while the wide-band power splitter 30 is reliable, a pig-tailing of the fourth terminal thereof, and the use of a filter 32 or etalon 42, lead to lower reliability and a high cost in addition to making the wave-locker arrangement 20 wavelength dependent.
It is desirable to provide a wavelength locking device for a laser diode which is wavelength independent, is economical and easy to fabricate, and provides for the absolute wavelength stability of the laser diode.
The present invention is directed to method and apparatus for providing a wavelength locking device which is wavelength independent and provides for the absolute wavelength stability of a laser diode.
Viewed from one aspect, the present invention is directed to a wavelength-independent wavelength-locker arrangement for controlling the output wavelength of a laser diode. The wavelength-locker arrangement comprises a narrow-band power splitter, first and second photodetectors, and a control unit. The narrow-band power splitter is responsive to the reception of a portion of an optical output signal from the laser diode for splitting the received optical signal into first and second coupler optical output signals, respectively. The first and second photodetectors are responsive to the reception of the first and second coupler optical output signals, respectively, for generating respective corresponding first and second electrical output signals. The control unit is responsive to the reception of the first and second electrical output signals from the first and second photodetectors for generating predetermined control signals to the laser diode. The control signals from the control unit maintain the wavelength of the laser diode at a predetermined value.
Viewed from another aspect, the present invention is directed to a wavelength-independent wavelength-locker arrangement for controlling the output wavelength of a laser diode. The wavelength-locker arrangement comprises an optical power tap, a narrow-band power splitter, first and second photodetectors, and a control unit. The optical power tap is responsive to an optical output signal from the laser diode for is splitting the optical output signal into first and second output optical signal portions. The first output optical signal portion is coupled into a first output optical fiber as an output signal from the wavelength-locker arrangement, and the second output optical signal portion is coupled into a second output optical fiber. The narrow-band power splitter is responsive to the reception of the second output optical signal portion from the optical power tap propagating in the second optical fiber for splitting the received second optical signal portion into first and second coupler optical output signals. The first and second photodetectors are responsive to the direct reception of the first and second coupler optical output signals, respectively, for generating respective corresponding first and second electrical output signals. The control unit is responsive to the reception of the first and second electrical output signals from the first and second photodetectors for generating predetermined control signals to the laser diode for maintaining the wavelength of the laser diode at a predetermined value.
Viewed from still another aspect, the present invention is directed to method of controlling the output wavelength of a laser diode in an wavelength-independent wavelength-locker arrangement. In a first step, a portion of an optical signal received from the laser diode at an input of a narrow-band power splitter is split into first and second coupler optical output signals. In a second step, first and second electrical output signals are generated in first and second photodetectors, respectively, from the respective first and second coupler optical output signals received directly from the narrow-band power splitter. In a third step, predetermined control signals are generated in a control unit in response to the reception by the control unit of the first and second electrical output signals from the first and second photodetectors, respectively, for maintaining the wavelength of the laser diode at a predetermined value.
The invention will be better understood from the following more detailed description taken with the accompanying drawings and claims.